Chromosomal translocations target master regulatory genes that affect growth, differentiation, and apoptosis. The t(8;21) is perhaps the most frequent chromosomal translocation associated with acute myeloid leukemia. This translocation fuses the first 177aa of RUNX1 (formerly known as AML1), including the DNA binding domain, to nearly all of Myeloid Translocation Gene on chromosome 8 (MTG8, also known as ETO). In addition to its involvement in acute leukemia, MTG8 was recently identified as a candidate cancer gene in colorectal carcinoma (CRC). The targeting of MTG8 in two tumor types emphasizes the critical role this gene plays in tumorigenesis. Studies of the t(8;21) fusion protein indicate that MTG8 functions as a transcriptional co-repressor. This application is focused on defining the role that this master regulator of transcription plays in tumorigenesis. We will determine how the mutations of MTG8 that are associated with colorectal cancer affect its function both in vitro and in vivo. Biochemical approaches will define how these mutations affect MTG8 recruitment of co- repressors and histone deacetylases and how these changes alter transcriptional repression. We will also take advantage of Mtg8-deficient mice to define the physiological contributions of this gene to gut development. We will also test whether inactivation of this gene stimulates tumorigenesis and determine how the CRC-related mutations affect its functions in the intestinal epithelium. Finally, we will investigate the molecular mechanisms underlying the ability of the t(8;21) fusion protein to disrupt transcriptional programs regulated by endogenous MTG family members and the nuclear hormone co-repressors. This information will not only shed light on the molecular mechanism by which MTG8 contributes to AML, but will also provide insights to its role in colorectal carcinoma.